Electronic flash system with control of commutation capacitor

ABSTRACT

An electronic flash including a discharge flash tube controlled by a first switch and a commutation capacitor for controlling the first switch. There is provided a second switch for governing the operation of the commutation switch. A charging circuit is provided for charging the commutation capacitor and includes a resistor connecting the commutation capacitor with an electric power source. The resistor has a resistance value which is small enough to allow a current not smaller than a current at which the second switch is turned off.

DESCRIPTION OF THE INVENTION

The present invention relates to an electronic flash system for photographing cameras, and more particularly to an electronic flash system capable of repeated flashing operations.

Electronic flash systems are in wide use not only in ordinary cameras but also in various cameras designed for specific purposes and there are demands for electronic flash systems which can be operated repeatedly with very short intervals. For example, in fluuorescent photographing of a retina of a patient's eye wherein fluorescent agent is injected to the patient and photographs of the retina are repeatedly taken with predetermined time intervals to record diffusion of the fluorescent agent, it is required to use flashing systems of strong illumination capable of repeating operations 3 to 5 times in a second.

In conventional electronic flash systems, a control of a flashing time is performed utilizing the commutation of a commutation capacitor of which capacity is determined by the quantity of electric current which flows through the flashing tube. In case of the fluorescent photographings of a retina, since it is required to make a large quantity of electric current to flow through the flashing tube, the commutation capacitor must be of a large capacity. On the other hands, such commutation capacitor having a large capacity may have a problem of being insufficiently charged when the flash system is operated very frequently, for example, 3 to 5 times a second, although it is required that the commutation capacitor be fully charged in order to perform an accurate control of the flashing time.

It is therefore an object of the present invention to provide an electronic flash system which can be operated repeatedly with very short time intervals.

Another object of the present invention is to provide an electronic flash which can be operated with a high accuracy of flashing time control even under a repeated operations with very short time intervals.

According to the present invention, the above and other objects can be accomplished by an electronic flash system including a flashing circuit having flashing means for producing a flash of illuminating light when energized and first switching means connected in series with said flashing means, a first bypass circuit having first capacitor means and second switching means connected in series with said first capacitor means, said first bypass circuit being connected in parallel with said first switching means for applying a reverse voltage to said first switching means to thereby turn off the first switching means so that flashing operation of the flashing means is terminated, a charging circuit for charging said first capacitor means, said charging circuit having first resistance means connected in series with said second switching means and second resistance means connected in parallel with said first switching means, a second bypass circuit for applying a reverse voltage to said second switching means to turn off said second switching means, said second bypass circuit having second capacitor means and third switching means connected in parallel with said second switching means, said first resistance means having a resistance value which allows to flow therethrough a current not smaller than a current at which the second switching means is turned off. According to the feature of the present invention, the first resistance means has a low resistance value so that the charge of the commutation capacitor can be carried out rapidly. However, the large quantity of current which is allowed to pass through the first resistance means and the second switching means will maintain the second switching means in the conductive state. Therefore, according to the present invention, there is provided the second bypass circuit to turn off the second switching means. With this arrangement of the present invention, it becomes possible to operate the flash system repeatedly at very short time intervals, for example, 3 to 5 times in a second.

The above and other objects and features of the present invention will become apparent from the following descriptions of preferred embodiments taking reference to the accompanying drawings, in which;

FIG. 1 is a circuit diagram showing an electronic flash system in accordance with one embodiment of the present invention;

FIG. 2 is a circuit diagram showing a control circuit for controlling the operation of the system shown in FIG. 1;

FIG. 3 is a diagram showing signals produced in the control circuit; and

FIG. 4 is a circuit diagram similar to FIG. 1 but showing another embodiment of the present invention.

Referring now to the drawings, particularly to FIG. 1, the electronic flash system shown therein comprises a main circuit 10, a flashing circuit 20, a first bypass circuit 30, a charging circuit 40, a second bypass circuit 50 and a trigger circuit 60. The main circuit 10 is provided for supplying electric power to the other circuits and includes an electric power source E and a main capacitor C₁ connected in parallel with the power source E. Connected with the power source E and the main capacitor C₁ in series is an inductance coil L₁ which is parallel with a diode D₁. The main capacitor C₁ functions to supply a large quantity of electric current to the flashing circuit 20 when a flash of illuminating light is being produced. Thus, it is necessary that the capacity of the main capacitor C₁ be determined in accordance with the power consumption of the flashing circuit 20. The inductance coil L₁ is provided for suppressing momentary currents and the diode D₁ for preventing a backlash.

The flashing circuit 20 includes a discharge tube X_(E) having one pole connected with the induction coil L₁ in the main circuit 10 and the other pole grounded through a first thyristor SCR₁. The thyristor SCR₁ has a triggering gate which is connected through a resistor R₁ with a triggering input terminal X. The first bypass circuit 30 is provided for turning off the thyristor SCR₁ to de-energize the flashing tube X_(E) and includes a commutation capacitor C₂ and a thyristor SCR₂ connected in series with the capacitor C₂. The thyristor SCR₂ has a triggering gate connected with an input terminal Y and grounded through a resistor R₂.

The charging circuit 40 is provided for charging the capacitor C₂ and include a resistor R₃ having one end connected with the coil L₁ of the main circuit 10 and the other end connected with the capacitor C₂ at an end connected to the thyristor SCR₂. The charging circuit 40 further includes a resistor R₄ having one end connected with the other end of the capacitor C₂. The other end of the resistor R₄ is grounded. The resistor R₃ has a resistance value which is sufficiently low so that it allows an electric current to flow through the thyristor SCR₂ when the latter is turned on, with a current level substantially equal to or greater than the holding current which is inherent to the thyristor. The aforementioned other end of the capacitor C₂ is connected to the thyristor SCR₁ at the end connected to the flashing tube X_(E). The resistor R₄ has a resistance value which is sufficient to apply a reverse voltage from the commutation capacitor C₂ to the thyristor SCR₁.

The second bypass circuit 50 includes at transistor Q₁ having a collector connected through a resistor R₅ with the coil L₁. The collector of the transistor Q₁ is also connected through a capacitor C₃ with the capacitor C₂ and the thyristor SCR₂. The transistor Q₁ has a base grounded through a resistor R₆ and an emitter which is directly grounded. The base of the transistor Q₁ is connected with an input terminal Z. The second bypass circuit 50 functions to apply a reverse voltage from the capacitor C₃ to the thyristor SCR₂ when the transistor Q₁ is turned on to thereby turn off the thyristor SCR₂.

The trigger circuit 60 is provided for producing a high voltage for triggering the flash tube X_(E), and includes a transformer L₂ having a primary coil connected at one end with one end of a capacitor C₄, the other end of the primary coil being grounded. The capacitor C₄ has the other end connected on one hand through a resistor R₇ with the coil L₁ of the main circuit 10 and on the other hand with a thyristor SCR₃. The thyristor SCR₃ has a triggering gate connected through a resistor R₉ with the input terminal X. The resistor R₉ is grounded through a resistor R₈. The secondary coil of the transformer L₂ is connected with the triggering electrode of the flash tube X_(E).

In operation of the circuit shown in FIG. 1, the thyristors SCR₁, SCR₂ and SCR₃ and the transistor Q₁ are all turned off prior to operation. The capacitors C₁, C₂ and C₃ are charged with the polarities shown in FIG. 1. For initiating the flashing operation, a trigger pulse is applied to the input terminal X so that the thyristors SCR₁ and SCR₃ are turned on. Thus, the capacitor C₄ discharges through a loop comprised of the thyristor SCR₃ and the primary coil of the transformer L₂ to produce a high voltage at the secondary coil of the transformer L₂. Thus, a high voltage is applied to the triggering electrode of the flash tube X_(E) to initiate a discharge in the flash tube X_(E). The discharge current through the flash tube X_(E) flows through the thyristor SCR₁.

After a predetermined time, a trigger pulse is applied to the input terminal Y so that the thyristor SCR₂ is turned on. Thus, the current through the flash tube X_(E) is allowed to pass through the capacitor C₂ to the thyristor SCR₂ decreasing the potential at the anode of the thyristor SCR₁. This turns off the thyristor SCR₁ to thereby de-energize the flash tube X_(E). When the thyristor SCR₂ is thus turned on, the potential at the anode of the thyristor SCR₂ is decreased and the capacitor C₃ is charged with the polarity shown in FIG. 1.

When a predetermined time is passed after the thyristor SCR₂ is turned on, a trigger pulse is applied to the input terminal Z to turn the transistor Q₁ on. The predetermined time is determined taking into consideration the time required for having the thyristor SCR₁ turned off and the capacitor C₃ charged to a level sufficient to turn the thyristor off.

When the transistor Q₁ is turned on, the potential at the anode of the thyristor SCR₂ is decreased to turn off the thyristor SCR₂. At this instance, since the capacitor C₂ is charged with a polarity opposite to the shown in FIG. 1 so that the capacitor C₂ is now started to be charged by a current through the resistors R₃ and R₄ to the polarity shown in FIG. 1. Since the resistor R₃ has a resistance value which is sufficiently low so as to permit a current flow through the thyristor SCR₂ when it is turned on, at a level substantially equal to or larger than the holding current, the charging of the commutation capacitor C₂ is completed in a relatively short period. Thus, it becomes possible to operate the system repeatedly with very short intervals.

Referring now to FIG. 2, the control circuit shown therein includes a first signal section 100, a second signal section 200 and a third signal section 300. The first signal section 100 includes a mono-stable multivibrator IC₁ which is connected with a resistor R₁₁ and a capacitor C₅ for determining the pulse width, and a buffer amplifier 110 connected with the multivibrator IC₁. The buffer amplifier 110 includes a transistor Q₂ having a collector connected through a resistor R₁₄ with a voltage source and a base connected through a resistor R₁₂ with the multivibrator IC₁ and grounded through a resistor R₁₃. Further, the transistor Q₂ has an emitter leading to the input terminal X of the electronic flash system. The first signal section 100 further includes a switch SW and a resistor R₁₀ connected with the resistor R₁₁ and the voltage source. The switch SW may be interconnected with a shutter release button of a camera so that it is closed at the time of photographing. The voltage at the switch SW is shown in FIG. 3(1). The multivibrator IC₁ is applied with an input as shown in FIG. 3(1) to start its operation and produces a first control signal a shown in FIG. 3(2). It is desirable that the first control signal be of a short duration so that a termination of the flashing operation is not disturbed.

The first control signal is amplified by the amplifier 110 and passed to the terminal X to initiate the flashing operation. In FIG. 3(3), there is shown the discharge current through the flash tube X_(E). The output of the multivibrator IC₁ is also connected with the second signal section 200.

The section 200 includes a monostable multivibrator IC₂ having an input connected with the output of the multivibrator IC₁. The multivibrator IC₂ is associated with a resistor R₁₅ and a capacitor C₆ so that an appropriate pulse width is determined. The multivibrator IC₂ has an output connected with a multivibrator IC₃ which is associated with a resistor R₁₆ and a capacitor C₇ so that an appropriate pulse width is determined. The multivibrator IC₃ has an output which is connected with a buffer amplifier 210. The amplifier 210 includes a transistor Q₃ having a collector connected through a resistor R₁₉ with the voltage source and a base connected through a resistor R₁₇ with the output of the multivibrator IC₃. The base of the transistor Q₃ is also grounded through a resistor R₁₈. The transistor Q₃ further has an emitter leading to the input terminal Y of the flash system.

The multivibrator IC₂ is applied with the first control signal and produces a pulse signal having a duration corresponding to the flashing time as shown in FIG. 3(4). In order that the pulse duration be adjusted, the resistor R₁₅ is of a variable resistance. The multivibrator IC₃ receives the pulse output from the multivibrator IC₂ and produces a second control signal as shown in FIG. 3(5) which is used for terminating the flashing operation. The duration of the second control signal should be of a suitable value so that the thyristor SCR₂ be turned off without fail.

The output of the multivibrator IC₃ is also applied to the third signal section 300 which includes a mono-stable multivibrator IC₄ having an input connected with the output of the multivibrator IC₃. The multivibrator IC₄ is associated with a resistor R₂₀ and a capacitor C₈ so that an appropriate pulse duration is determined. The multivibrator IC₄ has an output connected with a mono-stable multivibrator IC₅ which is associated with a resistor R₂₁ and a capacitor C₉ so that an appropriate pulse width is determined.

The multivibrator IC₄ receives the second control signal from the multivibrator IC₃ and produces a pulse as shown in FIG. 3(6). The multivibrator IC₅ receives the pulse from the multivibrator IC₄ and produces a third control signal shown in FIG. 3(7). The output of the multivibrator IC₅ is connected to a buffer amplifier 310 having a transistor Q₄ and resistor R₂₂, R₂₃ and R₂₄. The output of the amplifier 310 is connected to the input terminal Z.

Referring to FIG. 4, the circuit shown therein is identical with that shown in FIG. 1 except a discharge promoting circuit 70 connected in parallel with the capacitor C₂. The circuit 70 includes a thyristor SCR₄ and a resistor R₃₀ which are connected in series with each other. The thyristor SCR₄ and the resistor R₃₀ are connected in parallel with the capacitor C₂. The thyristor SCR₄ has a triggering gate connected through a resistor R₃₁ with the capacitor C₂ at the end connected with the thyristor SCR₂. In parallel with the resistor R₃₁, there is a capacitor C₁₀. Further, series connected zener diode D₂ and diode D₃ are connected in parallel with the capacitor C₁₀. A resistor R₃₂ is connected in parallel with the thyristor SCR₄.

With this arrangement, by appropriately determining the resistance values of the resistors R₃₁ and R₃₂ and the capacitance of the capacitor C₁₀, it is possible to have the thyristor SCR₄ turned on at a timing close to the timing wherein the flashing operation is terminated and the transistor Q₁ is turned on. Then, the thyristor SCR₄ is turned on when the charge opposite to the polarity shown in FIG. 4 still remains in the capacitor C₂ so that the discharge of the capacitor C₂ is promoted. The circuit 70 does not have any influence when the capacitor C₂ is being charged to the polarity shown in FIG. 4.

The invention has thus been shown and described with reference to specific embodiments, however, it should be noted that the invention is in no way limited to the details of the illustrated arrangements but changes and modifications may be made without departing from the scope of the appended claims. 

I claim:
 1. An electronic flash system including a flashing circuit having flashing means for producing a flash of illuminating light when energized and first switching means connected in series with said flashing means, a first bypass circuit having first capacitor means and second switching means connected in series with said first capacitor means, said first bypass circuit being connected in parallel with said first switching means for applying a reverse voltage to said first switching means to thereby turn off the first switching means so that flashing operation of the flashing means is terminated, a charging circuit for charging said first capacitor means, said charging circuit having first resistance means connected in series with said second switching means and second resistance means connected in parallel with said first switching means, a second bypass circuit for applying a reverse voltage to said second switching means to turn off said second switching means, said second bypass circuit having second capacitor means and third switching means which are connected in series with each other, said second bypass circuit being connected in parallel with said second switching means, said first resistance means having a resistance value which allows to flow therethrough a current not smaller than a current at which the second switching means is turned off.
 2. An electronic flash system in accordance with claim 1 which further includes means for promoting discharge of said first capacitor means when said first switching means is turned off.
 3. An electronic flash system including flashing means for producing a flash of illuminating light, said flashing means having first switching means for controlling operation of said flashing means, first bypass means including first capacitor means and second switching means which are connected in series with each other and connected in parallel with said first switching means, charging means connected with said first capacitor means so that said first capacitor means is charged in one polarity when said second switching means is turned off, first trigger means for turning on the first switching means and initiating operation of the flashing means, second trigger means for turning on the second switching means to make the first capacitor means discharge and apply to the first switching means a voltage with turns off the first switching means to thereby make current from the flashing means flow through said first capacitor means so that the capacitor means is charged in the opposite polarity, second bypass means having second capacitor means and third switching means which are connected together in series and adapted for applying a reverse voltage to the second switching means to turn-off the second switching means, third trigger means for turning off the second switching means, said charging means including resistor means connected between electric power source means and said first capacitor means and having a resistance value which allows a current not smaller than a current at which the second switching means is turned off.
 4. An electronic flash in accordance with claim 3 in which means is provided for promoting discharge of said first capacitor means after said first switching means is turned off so that the first capacitor means can be charged quickly to said one polarity when the second switching means is turned off.
 5. An electronic flash system in accordance with claim 4 in which said promoting means includes third switching means connected in parallel with the first capacitor means. 